The invention relates to a method for producing a magnetic tunnel contact according to the preamble of claim 1 as well as a magnetic tunnel contact in accordance with the preamble of claim 4.
Up to now, ferroelectric and magnetoresistive storage technologies have been developed at high cost in which dynamic random access memory DRAM has been replaced by nonvolatile supply voltage independent storage components, designated as FRAM (ferroelectric random access memory) or MRAM (magnetic random access memory).
For MRAMs, various magnetoresistive effects can be contemplated. AMR (anisotropic magnetoresistance) storage is based upon the fact that the resistance to electrical currents in a direction parallel to the magnetization direction of a conductive material and perpendicular to the magnetization direction differ. The GMR (giant magnetoresistance) storage concept is based upon the giant magnetoresistance effect which arises in layer stacks comprised of alternating magnetic layers, of nanometer thinness, e.g. of cobalt, and nonmagnetic layers (e.g. of copper). A third candidate for MRAMs are magnetic tunnel contacts. They are comprised of two ferromagnetic layers which are separated by a thin tunnel insulated layer.
A further field of use for components based upon the magnetoresistive effect is the magnetic field sensor technology. To date AMR magnetic field sensors, GMR magnetic field sensors and tunnel magnetic field sensors are known.
The manner in which magnetic tunnel contacts work rests upon a spin-dependent change of the tunnel resistance (TMR: tunneling magnetoresistance) for normally conducting electrons which is based upon a spin polarization of the participating magnetic layer. Until a brief time ago, this effect had only interest as a matter of principle since the relative change in the magnetoresistance xcex94R/R was less than 1% for the pin-dependent tunnels. The situation changed in 1995 with T. Miyazaki et al in xe2x80x9cGiant Magnetic Tunneling Effect in Fe/Al2O3/Fe Junctionxe2x80x9d J. Magn. Magn. Matter. 139, pages L231-L234, which was directed to magnetic tunnel contacts with xcex94R/R values xe2x89xa718% at room temperature.
A substantial advantage of the magnetic tunnel contact resides in that it is high ohmic and thus has greater compatibility with the existing integrated semiconductor circuits than the low ohmic AMR or GMR elements. Thus it is to be expected that magnetic tunnel contacts have the best perspective [sic: prospects] (as memory or sensor elements) for integration in coming generations of chips.
For the acceptability of a magnetic tunnel contact for use in integrated circuitry it is required that the magnetic tunnel contact have, apart from the previously discussed requirement, the greatest possible relative change in the magnetoresistance xcex94R/R, a resistance-area product RA which lies in a range from 102 to a maximum of about 104xcexa9xc3x97xcexcm2 optimally. Magnetic tunnel contacts which simultaneously satisfy these two requirements could not hitherto have been made.
In the publication representing the closest prior art xe2x80x9cMagnetic Tunneling Applied to Memoryxe2x80x9d, of J. M. Daughton, J. Appl. Phys. 81, pages 3758-3763 (1997), a magnetic tunnel contact is described with relative magnetoresistance change xcex94R/R of 20% and a resistance-area product RA of 10xe2x88x92106 kxcexa9xc3x97xcexcm2 is used. To produce the tunnel insulating layer a 2 nm thick partially oxidized Al layer and an 0.6 nm thick completely oxidized Al2O3 layer is used. It has been found to be advantageous to utilize an incompletely oxidized metal layer as the magnetic tunnel layer.
In the publication xe2x80x9cBias Voltage and Temperature Dependence of Magnetotunneling Effectxe2x80x9d, Yu Lu, et al, J. Appl. Phys., 83 Pages 6515-6517, (1998), a magnetic tunnel layer has been described which utilizes an oxidized aluminum layer as the tunnel barrier layer. To produce the oxidized barrier layer, a 0.5 to 1.5 nm thin aluminum layer is deposited on the lower ferromagnetic layer of the tunnel contact and then with a plasma oxidizing process is oxidized. Advantageously in the plasma oxidation method short oxidation times (30 seconds to 7 minutes) suffice to produce the insulating layer and it is possible to obtain a sufficiently high relative magnetoresistance (more than 16% at room temperature). It is however a drawback that the magnetic tunnel insulating layer produced with this process has too high a resistance-area product RA for integration in semiconductor chips.
In the publication xe2x80x9cPreparation of Nb/Alxe2x80x94AlOx/Nb Josephson Junctions with Critical Current Densities Down to 1 A/cm2xe2x80x9d of L. Fritzsch, et al., PHYSICA C 296, pages 319-324, (1998), a method for producing tunnel insulating layers is described for a superconductive Josephson tunnel contacts. In the process, an 8 nm electric aluminum layer is deposited on a niobium electrode and by means of a UV light supported oxidation process is completely oxidized to a 1.3 mm thick surface layer region. The Josephson tunnel contact must be operated at a temperature below the critical temperature (9.2 K) of the niobium and based upon the tunnels with Cooper pairs comprised of two paired electrons with opposite spin, the publication contributes nothing with respect to possible uses of such systems outside the range of superconductive tunnel contacts and especially can give no indication of its magnetic characteristics.
The object of the invention is to provide a process for producing a metallic tunnel contact which is comparable with different processes common in semiconductor technology and enables a magnetic tunnel contact to be obtained for integration in semiconductor circuits which has a satisfactory electrical characteristic. It is further a target of the invention to provide a magnetic tunnel contact such that the use thereof in future semiconductor circuits, especially in the form of a MRAM or a magnetic field sensor is appropriate.
The invention is attained with the features of claims 1 and 4.
Through the use of the UV light supported oxidation process, magnetic tunnel contacts can be obtained which at 300 K have a relative magnetoresistance of more than 10% (and at a first try up to 22%) and a resistance-area product RA of less than 10 kxcexa9xc3x97xcexcm2, especially less than 2 kxcexa9xc3x97xcexcm2, (and at the first tests up to 0.3 kxcexa9xc3x97xcexcm2) so that with the process according to the invention magnetic tunnel contacts with a good sensitivity (i.e. with a large relative magnetoresistance ratio xcex94R/R) and values for the resistance-area product RA can be made which are optimal for MRAM applications with respect to switching speed and compatibility for CMOS readout circuits. It has been found to be advantageous to carry out the process according to the invention so that the oxidation process on the one hand is merely a stoichiometric oxidation of the metal without any remaining nonoxidized metal component and, on the other hand, by contrast to the aggressive plasma oxidation or etching process which is known to the state of the art, a precise self-limiting oxidation stop is obtained at the boundary layer to the ferromagnetic material lying thereunder.
A further advantage of the process in accordance with the invention resides in that in spite of the relatively xe2x80x9ccleanxe2x80x9d oxidation it can be carried out sufficiently rapidly so that it can be integrated in the duration of the semiconductor fabrication process. Since metal depositions steps and oxidation steps have long been used in semiconductor technology, no difficulties arise in integration of the process into that process technology.
Advantageously in the deposition of the metal layer the coating thickness is adjusted to a maximum of about 2 nm. This ensures that the deposited metal layer will be completely oxidized (i.e. not only stoichiometrically but also through the total layer). It has been found that nonoxidized metal components in the insulation layer should be avoided to the extent possible since they produce lower values for the resistance-area product. The material for the metal layer is preferably Al, Mg, Hf, Ta, Zr, Y or Er.
A further advantageous feature is that the tunnel contact is structured by a lithographic step and an ion beam step in especially an elliptical form. Via the form anisotropy a magnetic easy access is defined whose effect is to capture for the magnetic domain a preferred direction along the longitudinal axis of the ellipse.
Of special advantage is the use of the magnetic tunnel contact according to the invention in a memory cell and/or in a magnetic field sensor.